Tool monitoring systems which monitor the acoustic emissions resulting from a machine tool cutting process are well known in the art. One type of known tool monitoring system employing acoustic emissions amplifies the acoustic emissions, detects the envelope of the amplified acoustic emissions and then converts the envelope to a signal representing the RMS value of the envelope. This RMS signal is compared with a threshold value to determine whether an alarm signal should be generated, wherein the alarm signal indicates an unacceptable cutting condition, as where the machine tool is badly worn or has broken.
Another type of tool monitoring system employing acoustic emissions utilizes a special type of signal processing in order to determine the rate of energy being generated by the cutting process. The resulting energy rate signal is compared with the threshold value to determine whether or not an alarm should be activated.
RMS and energy rate signal processing each have its advantages and may be particularly well-suited for a particular application. In using either technique described above, it has been found to be important to maintain the signal resulting from the acoustic emissions to within a prescribed range of values. If the signal is too high in magnitude, the processing circuitry may become saturated and thereby provide erroneous results. On the other hand, if the signal is too weak, the processing circuitry may not have sufficient sensitivity to properly process it, thereby diminishing the accuracy of the system. Although it may be possible to initially adjust the system gain for a given set of cutting conditions, the acoustic emission signal may have a wide dynamic range owing to dramatically changing cutting patterns, workpiece materials and different types of cutting tools. Consequently, an initial calibration of system gain for a given set of cutting conditions may not solve the problem. The problem mentioned above is likewise encountered in other types of tool monitoring systems which monitor other parameters which indicate the cutting condition of the tool, such as tool vibration, axial and radial tool force, etc.
Another problem associated with prior art monitoring systems such as those which monitor acoustic emissions is the difficulty in determining the point in time when actual cutting of the workpiece commences. It is important to determine when cutting commences so that the monitoring system can be properly initialized and activated.